Push rods for pistons in compression release engine retarders

Description

Background of the Invention

[0001] This invention relates to engine retarders of the compression release type. More particularly, the invention relates to an improved master piston for a compression release engine retarder.

[0002] Engine retarders of the compression release type are well-known in the art. In general, such retarders are designed temporarily to convert an internal combustion engine into an air compressor so as to develop a retarding horsepower which may be a substantial portion of the operating horsepower normally developed by the engine in its powering mode.

[0003] The basic design of the compression release engine retarder is disclosed in Cummins U.S. Patent No. 3,220,392. That design employs a hydraulic system wherein the motion of a master piston actuated by an intake, exhaust, or injector pushrod or rocker arm of the associated engine controls the motion of a slave piston which, in turn, opens the exhaust valve in an engine cylinder whose piston is near its top dead center position. As a result, the work done during the compression stroke of the engine piston is not recovered during the subsequent expansion or power stroke but, instead, is dissipated through the engine exhaust and cooling systems.

[0004] U.S. Patent 3,220,392 shows a master piston assembly for use in a compression relief engine retarder, comprising a piston member having a piston head at one end, and a shaft portion having a ball joint having a substantially spherical surface at the end remote from the piston head, the shaft portion extending from the piston head to the substantially spherical surface, the piston head having a diameter which is larger than the transverse dimension of said shaft portion; and a foot member swivelly mounted on said substantially spherical surface for contacting a component of an engine associated with the compression relief engine retarder which provides a mechanical input for operating the master piston assembly when the compression relief engine retarder is in operation, said substantially spherical surface is being convex.

[0005] U.S. Patent 4,666,330 shows that retaining means may be provided at a ball joint for retaining one member of the joint in any swivel position relative to another member of the joint to prevent detachment of said one member from said another member.

[0006] FR-A-1506178 shows a foot member swivelly mounted on a substantially spherical surface for contacting a component of an engine associated with the compression relief engine retarder which provides a mechanical input for operating the master piston assembly when the compression relief engine retarder is in operation.

[0007] Heretofore, master pistons have been complex assemblies that are costly to manufacture and are subject to wear and/or cause wear on the engine surface in contact with them. The rounded surface that makes contact with the pushrod or rocker arm is difficult and expensive to manufacture. In addition, the line contact between the master piston and the pushrod or rocker arm causes high stress at the contact line, which increases wear of the master piston and the pushrod or rocker arm. In addition, the prior art design of the return spring requires a hole to be bored into the piston for placement of the return spring, after which, the hole is closed with a threaded plug. The complex process required for the manufacture of the contact surface and the return spring cavity is very costly.

[0008] In view of the foregoing, it is an object of this invention to simplify the design of master pistons for compression release engine retarders, thereby reducing the manufacturing requirements and cost. It is a further object of this invention to improve the design of the contact region and return spring of master pistons to reduce cost and wear.

Summary of the Invention

[0009] These and other objects of the invention are accomplished in accordance with the principles of the invention by providing an improved master piston which can be manufactured without any complex surfaces or assemblies. The master piston assembly of the present invention is characterized in that the foot member is a substantially cylindrical element having a flat exterior surface at an end thereof opposite to the joint ball joint, for sliding in use on the component of the engine, and that resilient means is provided, biased radially inwardly against the spherical surface, for resiliently urging the foot member to remain in any swivel position relative to the substantially spherical surface in which the foot member is left after contact with that component of the engine.

[0010] The installation of the return spring is simplified to no longer require a bored cavity within the piston body, nor a threaded plug to close that cavity. The master piston contact region is separated from the master piston body and modified to provide automatic alignment between the master piston and the associated push rod or rocker arm, thereby eliminating the need for an anti-rotation pin. Additionally, the contact region is modified to provide increased contact area, thereby reducing contact stress between the master piston and the push rod or rocker arm.

[0011] Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.

Brief Description of the Drawings

[0012] FIG. 1 is a simplified elevation view, partly in section, of a prior art master piston assembly.

[0013] FIG. 2 is another view similar to FIG. 1 showing the apparatus of FIG. 1 rotated 90° about a vertical axis.

[0014] FIG. 3 is a simplified elevation view, partly in section, of an illustrative embodiment of the invention before contact with a rocker arm or other mechanical input element.

[0015] FIG. 4 is a view similar to FIG. 3 showing the apparatus of FIG. 3 after contact with a rocker arm or other mechanical input element.

Detailed Description of the Preferred Embodiments

[0016] FIGS. 1 and 2 show a typical prior art master piston assembly 10, which comprises master piston body 2, threaded plug 1, balancing groove 3 (which tends to evenly distribute hydraulic pressure to keep master piston assembly 10 concentric in master piston cylinder 6), return spring 4 located within cavity 9 of master piston body 2, and anti-rotation pin 7. Master piston assembly 10 is shown in its condition when the engine brake is off and therefore return spring 4 alone controls the vertical position of body 2 in cylinder 6. Anti-rotation pin 7 provides three functions to master piston assembly 10, the first of which is to keep master piston assembly 10 within master piston cylinder 6. The second function of anti-rotation pin 7 is to prevent master piston assembly 10 from rotating within master piston cylinder 6. This could cause curved contact surface 5 to become improperly oriented with respect to the surface of the engine push rod or rocker arm (not shown) on which surface 5 bears when the compression release engine brake is in operation. The final function of anti-rotation pin 7 is to restrict expansion of prestressed compression coil return spring 4.

[0017] In manufacturing the above assembly, there are various processes that increase the cost of manufacturing. Contact surface 5 is a rounded surface that is difficult to manufacture and therefore expensive. The shaping of master piston body 2 to receive anti-rotation pin 7 in elongated slot 8 is also expensive, as is the process of boring master piston body 2 to create cavity 9 for insertion of return spring 4. Assembly of master piston assembly 10 requires that return spring 4 be inserted into cavity 9, followed by screwing threaded plug 1 into master piston body 2. This completed assembly is then inserted into master piston cylinder 6 and anti-rotation pin 7 must then be screwed through cylinder 6 and slot 8 into piston body 2. This relatively complex assembly process further increases manufacturing cost.

[0018] In addition to increased manufacturing cost, rounded contact surface 5 makes a line contact with an associated push rod or rocker arm as mentioned above to receive engine timing inputs. Such a line contact provides a contact region between master piston assembly 10 and the associated push rod or rocker arm which is a relatively small area. This small contact area tends to promote wear of the master piston and/or the push rod or rocker arm.

[0019] In the illustrative embodiment of the present invention shown in FIGS. 3 and 4, master piston assembly 20 comprises master piston body 12, return spring 14, and foot 23. Master piston assembly 20 is again shown in its condition when the engine brake is off so that the position of body 12 is determined entirely by return spring 14. Master piston body 12 comprises at one end piston head 15 having annular balancing groove 13 (which tends to evenly distribute hydraulic pressure to keep master piston assembly 20 concentric in master piston cylinder 16) and at the other end spherical ball joint 17. Return spring 14 is a prestressed compression coil spring having one end in annular channel 18 and the other end abutting washer 21 which is held in place by snap ring 22.

[0020] Foot 23 is a cylindrical object with a closed end and an open end which can swivel on ball joint 17. (By "swivel" it is meant that foot 23 can both pivot and rotate with respect to ball joint 17.) The closed end of foot 23 has an exterior flat contact surface 25 and an interior spherical surface which complements the surface of ball joint 17. There are two annular slots 27 and 28 located adjacent the open end of foot 23. The foremost slot 27 holds retainer 24 which is a prestressed split ring spring, biased radially outward against the bottom of slot 27 to prevent detachment of foot 23 from ball joint 17. The lower slot 28 holds retention spring 26 which is also a prestressed split ring spring, but is biased radially inward against ball joint 17 to retain the angular alignment of foot 23 with the associated push rod or rocker arm after the compression release engine retarder has been turned off and return spring 14 lifts the master piston out of contact with the push rod or rocker arm. Master piston assembly 20 provides improvement over master piston assembly 10 in many ways as will be shown below.

[0021] In order to reduce manufacturing costs, contact surface 25 has been removed from master piston body 12 and redesigned. Contact surface 25 is now a flat surface, which is easier to manufacture than curved contact surface 5 on master piston body 2, and therefore less costly. The improved design of contact surface 25 eliminates the need to prevent rotation of master piston assembly 20 within master piston cylinder 16, thereby eliminating one of the needs for an anti-rotation pin. Another improvement with respect to manufacturing is the installation of return spring 14 into master piston assembly 20. Unlike master piston assembly 10, the installation of return spring 14 does not require the additional manufacturing steps of boring a cavity into a master piston body and screwing a threaded plug into the master piston body. Return spring 14 is installed into channel 18 during installation of master piston body 12 into master piston cylinder 16, and is held in place by washer 21 and snap ring 22 which snaps into an annular slot in the wall of master piston cylinder 16. The new installation of return spring 14 eliminates the remaining need for an anti-rotation pin and its associated elongated slot, thereby further reducing manufacturing and assembly costs.

[0022] Master piston assembly 20 also provides an increase in reliability over master piston assembly 10 by eliminating the anti-rotation pin which may be subject to shear failure. FIG. 4 illustrates a further improvement in reliability showing the functionality of redesigned contact surface 25. Master piston assembly 20 provides a flat contact surface 25, thereby increasing the area of contact with the associated push rod or rocker arm. Flat contact surface 25 reduces contact stress between master piston assembly 20 and the push rod or rocker arm, thereby reducing wear on both components. Master piston assembly 10 requires a curved contact surface 5 to maintain alignment with the associated push rod or rocker arm. Master piston assembly 20 uses retention spring 26 to hold foot 23 in position (as shown in Fig. 4) after the compression release engine retarder has been turned off and return spring 14 has removed master piston assembly 20 from contact with the push rod or rocker arm. Foot 23 is therefore in the proper position for contacting the push rod or rocker arm when the engine retarder is turned on again.

[0023] A different size foot providing increased or reduced surface area can easily be installed to accommodate different engine configurations, push rods, or rocker arms.

Claims

1. A master piston assembly for use in a compression relief engine retarder, comprising a piston member having a piston head (15) at one end, and a shaft portion (12) having a ball joint having a substantially spherical surface (17) at the end remote from said piston head (15), said shaft portion (12) extending from said piston head to said substantially spherical surface (17), said piston head (15) having a diameter which is larger than the transverse dimension of said shaft portion (12); a foot member (23) swivelly mounted on said substantially spherical surface (17) for contacting a component of an engine associated with said compression relief engine retarder which provides a mechanical input for operating the master piston assembly when the compression relief engine retarder is in operation, said substantially spherical surface (17) being convex; and retaining means (24, 27) provided between said foot member (23) and said substantially spherical convex surface (17) for retaining said foot member in any swivel position relative to said substantially spherical convex surface (17) to thereby prevent detachment of said foot member (23) from said substantially spherical convex surface, characterized in that said foot member is a substantially cylindrical element having an exterior flat surface (25) at an end thereof opposite to said ball joint, for sliding in use on said component of said engine, and that resilient means (26, 28) is provided, biased radially inwardly against said spherical surface (17), for resiliently urging said foot member (23) to remain in any swivel position relative to said substantially spherical surface (17) in which said foot member is left after contact with said component of said engine.

2. The master piston assembly defined in claim 1, further comprising means (14) for resiliently urging said piston member to move in the direction of said piston head (15) relative to a master piston cylinder (16) in which said piston member (12) is disposed.

3. The master piston assembly defined in claim 2, wherein said member for resiliently urging comprises:
   a prestressed compression coil spring (14) disposed around said shaft portion (12).

4. The master piston assembly defined in claim 3, wherein one end of said prestressed compression coil spring (14) bears on said piston head (15) and the other end of said prestressed compression coil spring acts on said master piston cylinder (16).

5. The master piston assembly defined in claim 4, wherein said one end of said prestressed compression coil spring is received in an annular recess (18) in said piston head (15).

6. The master piston assembly defined in claim 4, further comprising a snap ring (22) disposed in an annular slot in the wall of said master piston cylinder so that said snap ring (22) projects radially inward from the wall of said cylinder (16) adjacent said shaft portion (12), and wherein said other end of said prestressed compression coil spring (14) acts on said master piston cylinder (16) via said snap ring (22).

7. The master piston assembly defined in claim 6, further comprising a washer (21) interposed between said other end of said prestressed compression coil spring (14) and said snap ring (22).

8. The master piston assembly defined in claim 1, wherein said resilient means for resiliently urging comprises:

an annular recess (28) in a wall of said foot member (23) adjacent said substantially spherical surface (17); and

a split ring spring (26) disposed in said annular recess (28) and resiliently biased into contact with said spherical surface (17).

9. The master piston assembly defined in claim 1, wherein said piston head (15) has a front and a back having a bored axially extending annular channel (18), and wherein a washer (21), a snap ring (22), and a prestressed compression coil return spring (14) are provided, said spring (14) having one end located in said channel (18) and another end held in place by said washer (21), said washer (21) being held in place by said snap ring (22) which holds said master piston assembly in a master piston cylinder (16), said foot (23) having means (28, 26) for self-adjust alignment between said foot (23) and an associated engine push rod or rocker arm.

10. The master piston assembly defined in claim 9, wherein said means for self-adjust alignment comprises:
   a prestressed split ring retention spring (26), inwardly biased against said ball joint to retain said alignment of said foot with respect to said associated engine push rod or rocker arm after said compression release engine retarder has been turned off; and said retaining means (24, 27) includes a prestressed split ring retention spring (24), outwardly biased against said foot (23) to prevent detachment of said foot (23) from said substantially spherical convex surface (17).

Ansprüche

1. Hauptkolbenanordnung zum Gebrauch in einer Kompressionsfreigabe-Motorbremse, umfassend ein Kolbenelement mit einem Kolbenkopf (15) an einem Ende und einem Wellenbereich (12) mit einem Kugelgelenk, das am vom Kolbenkopf (15) entfernten Ende eine im wesentlichen sphärische Oberfläche (17) aufweist, welcher Wellenbereich (12) sich vom Kolbenkopf zu der im wesentlichen sphärischen Oberfläche (17) erstreckt, welcher Kolbenkopf (15) einen Durchmesser aufweist, der größer ist als die Querdimension des Wellenbereiches (12) ; ein Fußelement (23), das schwenkbar an der im wesentlichen sphärischen Oberfläche (17) angebracht ist, um eine Komponente eines mit der Kompressionsfreigabe-Motorbremse verbundenen Motors zu berühren, die eine mechanische Eingangsgröße zum Betreiben der Hauptkolbenanordnung bereitstellt, wenn die Kompressionsfreigabe-Motorbremse in Betrieb ist, wobei die im wesentlichen sphärische Oberfläche (17) konvex ist; und Rückhaltemittel (24, 27), die zwischen dem Fußelement (23) und der im wesentlichen sphärischen, konvexen Oberfläche (17) vorgesehen sind, um das Fußelement in jeder Schwenkposition in Bezug auf die im wesentlichen sphärische, konvexe Oberfläche (17) zurückzuhalten, um dadurch eine Ablösung des Fußelements (23) von der im wesentlichen sphärischen, konvexen Oberfläche zu verhindern, dadurch gekennzeichnet, daß das Fußelement ein im wesentlichen zylindrisches Element ist, das eine äußere, flache Oberfläche (25) an seinem dem Kugelgelenk entgegengesetzten Ende aufweist, um bei Gebrauch auf der Komponente des Motors zu gleiten, und daß ein federndes Mittel (26, 28) vorgesehen ist, das gegen die sphärische Oberfläche (17) radial einwärts vorgespannt ist, um das Fußelement (23) federnd vorzudrücken, um in jeder Schwenkposition relativ zu der im wesentlichen sphärischen Oberfläche (17) zu verbleiben, in der das Fußelement nach Berührung mit der Komponente des Motors gelassen wird.

2. Hauptkolbenanordnung gemäß Anspruch 1, ferner umfassend Mittel (14) zum federnden Vordrücken des Kolbenelements, um sich in Richtung des Kolbenkopfes (15) in Bezug auf einen Hauptkolbenzylinder (16) zu bewegen, in dem das Kolbenelement (12) angeordnet ist.

3. Hauptkolbenanordnung gemäß Anspruch 2, wobei das Element zum federnden Vordrücken umfaßt:
   eine vorgespannte Druckspiralfeder (14), die um den Wellenbereich (12) angeordnet ist.

4. Hauptkolbenanordnung gemäß Anspruch 3, wobei ein Ende der vorgespannten Druckspiralfeder (14) auf dem Kolbenkopf (15) aufliegt und das andere Ende der vorgespannten Druckspiralfeder auf den Hauptkolbenzylinder (16) wirkt.

5. Hauptkolbenanordnung gemäß Anspruch 4, bei der ein Ende der vorgespannten Druckspiralfeder von einer ringförmigen Ausnehmung (18) in dem Kolbenkopf (15) aufgenommen ist.

6. Hauptkolbenanordnung gemäß Anspruch 4, ferner umfassend:
   einen Sprengring (22), der in einem ringförmigen Schlitz in der Wand des Hauptkolbenzylinders angeordnet ist, so daß der Sprengring (22) von der Wand des Zylinders (16) an den Wellenbereich (12) angrenzend radial einwärts hervorragt, und wobei das andere Ende der vorgespannten Druckspiralfeder (14) über den Sprengring (22) auf den Hauptkolbenzylinder (16) wirkt.

7. Hauptkolbenanordnung gemäß Anspruch 6, ferner umfassend:
   eine Scheibe (21), die zwischen dem anderen Ende der vorgespannten Druckspiralfeder (14) und dem Sprengring (22) zwischengeordnet ist.

8. Hauptkolbenanordnung gemäß Anspruch 1, bei der das federnde Mittel zum federnden Vordrücken umfaßt:

eine ringförmige Ausnehmung (28) in einer Wand des Fußelements (23) an die im wesentlichen sphärische Oberfläche (17) angrenzend; und

eine Spaltringfeder (26), die in der ringförmigen Ausnehmung (28) angeordnet und federnd in Kontakt mit der sphärischen Oberfläche (17) vorgespannt ist.

9. Hauptkolbenanordnung gemäß Anspruch 1, bei der der Kolbenkopf (15) eine Vorderseite und eine Rückseite aufweist, die einen gebohrten, sich axial erstreckenden, ringförmigen Kanal (18) besitzt, und bei der eine Scheibe (21), ein Sprengring (22) und eine vorgespannte Druckspiral-Rückkehr-feder (14) vorgesehen sind, wobei die Feder (14) mit einem Ende in dem Kanal (18) angeordnet und mit dem anderen Ende durch die Scheibe (21) in Position gehalten wird, wobei die Scheibe (21) durch den Sprengring (22) in Position gehalten wird, der die Hauptkolbenanordnung in einem Hauptkolbenzylinder (16) hält, wobei der Fuß (23) Mittel (28, 26) zur selbsteinstellenden Ausrichtung zwischen dem Fuß (23) und einer zugehörigen Motorschubstange oder Kurbelschwinge aufweist.

10. Hauptkolbenanordnung gemäß Anspruch 9, bei der das Mittel zur selbsteinstellenden Ausrichtung umfaßt:
   eine vorgespannte Spaltring-Rückhaltefeder (26), die gegen das Kugelgelenk einwärts vorgespannt ist, um die Ausrichtung des Fußes in Bezug auf die zugehörige Motorschubstange oder Kurbelschwinge zu erhalten, nachdem die Kompressionsfreigabe-Motorbremse ausgeschaltet worden ist, und das Rückhaltemittel (24, 27) enthält eine vorgespannte Spaltring-Rückhaltefeder (24), die auswärts gegen den Fuß (23) vorgespannt ist, um ein Ablösen des Fußes (23) von der im wesentlichen sphärischen, konvexen Oberfläche (17) zu verhindern.

Revendications

1. Ensemble de piston maître destiné à être utilisé dans un ralentisseur de moteur à suppression de la compression, comprenant un piston ayant une tête (15) à une extrémité et une tige (12) ayant une rotule, présentant une surface sensiblement sphérique (17), à son extrémité éloignée de la tête (15) du piston, la tige (12) s'étendant à partir de la tête du piton jusqu'à la surface sensiblement sphérique (17), la tête (15) du piston ayant un diamètre qui est plus grand que la dimension transversale de la tige (12), un pied (23) monté à rotation sur la surface sensiblement sphérique (17) et destiné à venir en contact avec un composant d'un moteur associé au ralentisseur de moteur à suppression de la compression et qui fournit une entrée mécanique pour actionner l'ensemble de piston maître lorsque le ralentisseur de moteur à suppression de la compression est en fonctionnement, la surface sensiblement sphérique (17) étant convexe, et des moyens de retenue (24,27) prévus entre le pied (23) et la surface sensiblement sphérique (17) afin de retenir le pied dans n'importe quelle position en rotation par rapport à la surface sensiblement sphérique (17), afin d'éviter ainsi une séparation du pied (23) à partir de la surface sensiblement sphérique (17), caractérisé en ce que le pied est un élément sensiblement cylindrique ayant une surface externe plane (25) à son extrémité opposée à la rotule, cette surface étant destinée à glisser, en cours d'utilisation, sur le composant du moteur, et en ce qu'il est prévu un moyen élastique (26,28), sollicité radialement vers l'intérieur contre la surface sensiblement sphérique (17), afin de solliciter élastiquement le pied (23) de telle façon qu'il soit maintenu dans n'importe quelle position en rotation par rapport à la surface sensiblement sphérique (17) dans laquelle le pied est laissé après avoir été en contact avec le composant du moteur.

2. Ensemble de piston maître suivant la revendication 1 caractérisé en ce qu'il comprend en outre un moyen (14) pour solliciter élastiquement le piston de telle façon qu'il se déplace en direction de la tête (15) du piston par rapport à un cylindre (16) de piston maître dans lequel est disposé le piston (12).

3. Ensemble de piston maître suivant la revendication 2 caractérisé en ce que le moyen de sollicitation élastique est constitué par un ressort de compression hélicoïdal précontraint (14) disposé autour de la tige (12).

4. Ensemble de piston maître suivant la revendication 3 caractérisé en ce qu'une extrémité du ressort de compression hélicoïdal précontraint (14) est en appui contre la tête (15) du piston et l'autre extrémité du ressort de compression hélicoïdal précontraint (14) agit sur le cylindre (16) du piston maître.

5. Ensemble de piston maître suivant la revendication 4 caractérisé en ce qu'une extrémité du ressort de compression hélicoïdal précontraint (14) est logée dans un creux annulaire (18) prévu dans la tête (15) du piston.

6. Ensemble de piston maître suivant la revendication 4 caractérisé en ce qu'il comprend en outre un circlips (22) disposé dans une gorge annulaire formée dans la paroi du cylindre du piston maître de manière que ce circlips (22) fasse saillie radialement vers l'intérieur à partir de la paroi du cylindre (16), au voisinage de la tige (12), et en ce que l'autre extrémité du ressort de compression hélicoïdal précontraint (14) agit sur le cylindre (16) du piston maître par l'intermédiaire du circlips (22).

7. Ensemble de piston maître suivant la revendication 6 caractérisé en ce qu'il comprend en outre une rondelle (21) interposée entre l'autre extrémité du ressort de compression hélicoïdal précontraint (14) et le circlips (22).

8. Ensemble de piston maître suivant la revendication 1 caractérisé en ce que le moyen de sollicitation élastique est constitué par une gorge annulaire (28) formée dans une paroi du pied (23) en étant adjacente à la surface sensiblement sphérique (17), et un ressort annulaire fendu (26) disposé dans la gorge annulaire (28) et sollicité élastiquement de manière à être en contact avec la surface sensiblement sphérique (17).

9. Ensemble de piston maître suivant la revendication 1 caractérisé en ce que la tête (15) du piston comporte une partie avant et une partie arrière qui présente un canal annulaire (18) s'étendant axialement, et en ce qu'une rondelle (21), un circlips (22) et un ressort de compression hélicoïdal précontraint (14) sont prévus, ce ressort (14) ayant une extrémité logée dans le canal (18) et une autre extrémité maintenue en place par la rondelle (21), cette rondelle (21) étant maintenue en place par le circlips (22) qui maintient l'ensemble de piston maître dans un cylindre (16) de piston maître, le pied (23) comportant un moyen (28,26) pour obtenir un alignement à auto-ajustement entre le pied (23) et un poussoir ou un culbuteur associé du moteur.

10. Ensemble de piston maître suivant la revendication 9 caractérisé en ce que le moyen assurant l'alignement à auto-ajustement comprend un ressort annulaire de retenue fendu, précontraint (26), sollicité vers l'intérieur contre la rotule de manière à maintenir l'alignement du pied par rapport au poussoir ou au culbuteur associé du moteur après que le ralentisseur de moteur à suppression de la compression a été mis hors service, et le moyen de retenue (24,27) comporte un ressort de retenue annulaire fendu, précontraint (24), sollicité vers l'extérieur contre le pied (23), afin d'empêcher la séparation du pied (23) de la surface convexe sensiblement sphérique (17).

Drawing